again, open public venue 500,000 people, you can't create a fail safe environment. >> reporter: investigators have swept up a large amount of potential evidence including small bomb fragments and surveillance pictures and tape but we have to say it's too early to know if this attack was a work of a terror group, domestic or foreign, or the act of a lone wolf who was inspired to act out. charlie? >> bob orr, thanks. cities around the country increased security. with us now is rudy giuliani, mayor of new york city during the 9/11 attacks who consults with other cities on handling terror attacks and also john miller, nypd commissioner during mr. giuliani's tenure. this turns the clock back to 2001. whatever the thinking was on september 12th is now the thinking today. >> it really reminds us, right, of what we knew on september 11th and september 12th that the big news here is this is a horrible attack, terrible attack, my heart goes out to the people that were hurt but surprising there haven't been more of these since september 11th. we expected many attacks like this. the really remarkable sto